1. Field of the Invention
The present invention relates to a time-division multiplex digital transmission system in which data are transmitted in the form of multiplex pulse code modulation (PCM) words in time allocated to time slots forming a recurrent frame. The invention relates more particularly to a time-division multiplex digital transmission system of the kind comprising two terminal stations and at least one intermediate station connected to one another by way of two multi-channel multiplex lines corresponding to both transmission directions through intermediate stations in which some component channels transit without being multiplexed and other component channels are transferred, i.e. extracted from multiplex lines to local lines connected to the intermediate station, or inserted from local lines into free time slots of multiplex lines.
2. Description of the Prior Art
A time-division multiplex digital transmission system of the kind described, in which component channels may selectively transit through intermediate stations, or be transferred by coming out of a multiplex line or by entering into a multiplex line has been disclosed in the prior art, as e.g. in U.S. Pat. No. 3,987,250. To prevent data transmission from being cut off downstream of an intermediate station when the input data flow extracted in this station is cut off, each intermediate station is provided with a local time-base comprising an oscillator which can be synchronized by incoming timing bits extracted from a multiplex line. This oscillator, however, can continue to oscillate at a predetermined frequency when no incoming data signals are received from the multiplex line. This oscillator may be, for example, a voltage-controlled quartz oscillator (VCXO). The oscillator controls the single local time base which ensures, in the two data transmission directions on two multiplex lines respectively, framing, addressing for a time slot or channel in a frame and for a bit in a time slot, and controlling two multiplexers and demultiplexers, each being associated with one transmission direction. When the oscillator is no longer synchronized, it may shift, which may result in the local frame issued from local time base being out of phase with respect to a received frame. In this end, two buffer stores are provided, one for each transmission direction. Each buffer store for a given direction is controlled for write-in by timing bits extracted from the input data according to that direction, and for read-out by the single local time base.
This prior art time-division multiplex digital transmission system uses thus two multiplexers and demultiplexers for two data transmission directions respectively.